Side Effects
by Mini Nephthys
Summary: In which Issei does what anyone faced with an unexpected and unexplained gender change would do: act awkward around his - well, her - best friend. Shirou x genderswapped!Issei. Contest entry.


**Warnings: **Genderswaps, het that is usually slash except for said genderswaps, a little violence. That's it, because this is the censored version.

**Author's Notes:** Um, some of you may know of the Type-Moon Lemon Contest; the second round just ended. I entered this. It came in tied for second-to-last, with one vote.

Some of you may be wondering why this is rated T and only has about 5500 words when the contest required smut and had a 7000 word minimum. That's because this is the censored version. If you want to read the full version (a little more hopefully amusing dialogue, but also my fail attempts at smut), you'll have to search for "Type-Moon Lemon Contest 2". My entry is the one with the big **'Warning: Yaoi'** label. Can't miss it.

* * *

Knock knock.

Emiya blinked. Well, Sakura had said she was coming over earlier while Fuji-nee was away, but he didn't think she had meant at half past four… "Coming!" He headed over to the door and opened it. "You're here early, Saku- oh, Kuzuki-sensei? What are you doing here? And… who is that girl behind you?"

Said girl flinched. "…I guess I'm not entirely surprised, but I had hoped… well, I can't blame you, Emiya," she said, sounding slightly disappointed. "Kuzuki-sensei, could you please explain?"

"Certainly." Kuzuki looked completely calm when he continued, "Ryudo woke up this morning as a female."

Shirou stared. "…_What_?!"

Well, the glasses that girl wore looked the same as Issei's. And the poorly-fitting boy's school uniform she wore could have been Issei's. And they had the same color eyes and hair, and maybe the much longer hair was a side-effect of suddenly turning into a girl, but…

But you didn't just wake up the wrong gender one morning! This had to be some kind of joke…!

…even though Kuzuki probably never made a joke in his life, and that girl was flushing under his shocked stare like Issei always did…

"We thought it best if his father were not to find out," Kuzuki continued, apparently having decided that that was enough time for staring. "Would you be willing to take him in until I can discover the cause of this and the cure for it?"

"I hate imposing on you," the g- Issei added, looking at the ground rather than at him, "but our other options are Tohsaka and Matou, I would end up fighting with Tohsaka constantly, and Matou… well, if I stay with him, the possibility of harm to my person is not zero."

Shirou suddenly had a mental picture of Shinji looming over a terrified girl-Issei with an evil smirk on his face. "U-Um… It's okay with me if you stay here, but…" What was he going to tell Sakura and Ilya?

As if reading his mind, Issei replied, "I'll take all the blame with them for this. Thank you for being so understanding, Emiya."

Kuzuki nodded. "I will inform you if there is any progress." With that, he turned and left, probably heading back to the temple.

Shirou sighed, and then looked back at Issei. "…Come inside. Sakura should be here in a little while, so I'll explain things to them then…"

* * *

"That's all I know," Shirou finished, looking at his audience seated around the table.

Issei had her face buried in her hands. Sakura was glancing between Issei and Shirou, looking slightly pale. Ilya was just watching them all calmly.

Sakura was the first one to speak up. "I… Of course, I believe Kuzuki-sensei, but… do we have any other way of proving that this is really Ryudo-sempai?" she asked worriedly.

Issei said something unintelligible into her hands.

"What was that?" Shirou asked.

She lifted her head up. "The school's budget for the clubs gives ten percent of its funds to the archery club, seven point five percent to the track team, six point seven five-"

Sakura sighed with relief. "You're him."

"Are you going to be wearing that uniform the whole time you're here?" Ilya asked.

"It's the only clothing I have that even comes close to fitting me now," Issei answered. "…To be honest, the prospect of shopping for women's clothing frightens me…"

Ilya smiled brightly. "Don't worry! Sakura and I would be happy to help you find something!"

"We would?" Sakura asked, startled. "I mean… of course."

Issei blinked. "I… well, I don't have the money to afford the clothes-"

"I'll pay for them," Shirou interjected. "You look really uncomfortable in that."

"…If that's the reason, then I guess I have no choice but to be in your debt again, Emiya," she answered quietly.

* * *

"Oniichan, we're back~" Ilya called out, throwing open the door.

Shirou stepped out of the kitchen. "Great. Dinner's almost ready. Issei, you haven't changed?"

Issei set down her bags and raised an eyebrow. "You expected me to walk out of the store wearing the new clothes?" she asked. "I'll change into them after dinner - where should I put them? You haven't told me what room I'm sleeping in, and I… doubt I'm staying in your room like I did the last time I came over…" she said, blushing slightly and picking up her bags again.

"You can have Fuji-nee's room," he replied.

She nodded and disappeared in that direction.

"Since we picked out all her clothes, you have to give us the male opinion," Ilya told him.

"…Couldn't she give you the male opinion herself?" he asked. "I mean, since she used to be a guy and everything…"

"She's still panicking over being a woman," Sakura pointed out. "She doesn't want to worry you, so she's trying to act calm, but… It might help a little if you told her that she wasn't… um, what did she say she was?"

"'A grotesque mockery of her usual appearance,'" Ilya repeated as she rolled her eyes.

"Right. That."

"…I'll do my best," Shirou promised as Issei returned.

"Your best at what?" she asked.

"Making dinner. It's been a while since I cooked for you, Issei," he said somewhat honestly.

She smiled brightly. "I'm sure you'll do fine."

* * *

"It's okay, Issei. You can come out now," Shirou called out.

Issei's quavering voice came back. "I… Maybe you're making too much of an event out of this…"

"You can't stay in Fujimura-sensei's room forever," Sakura replied.

"Oniichan just wants to see you in the clothes he bought you before you turn back," Ilya added.

"I promise I won't say anything-" Shirou said, but stopped as Issei stepped into view.

He had known that his friend was _pretty_ like this since the moment he first saw her face. He thought all girls who wore glasses were _cute_.

But now that she stood in front of him, no longer in her badly-fitting uniform but wearing a sweatshirt and jeans, he could see perfectly her long legs, slim waist, and soft curves; he had to acknowledge that she was _attractive_.

"Well, I guess technically he isn't _saying_ anything…" Ilya muttered with a smirk.

"E-E-Emiya, you're staring…!" Issei squeaked out, her face starting to burn up. "Stop… stop looking at me like that…"

After a moment or two, Shirou snapped out of his stupor. "Sorry, Issei. You look nice."

"I… gathered that…" She took in an unsteady breath. "I'll just go back to my room now-!" And with that, she fled.

"…Should we have given her looser pants?" Ilya wondered aloud.

"Probably," Sakura agreed, still staring at where Issei had been.

Shirou sighed. "…um, if she's going to act like that every time she wears girls' clothes, won't living with her be kind of difficult…?"

* * *

Issei did not act like that every time she wore girl's clothes. Shirou didn't know whether it was because she'd gotten used to them or because he wasn't staring at her quite as hard. It was probably a bit of both.

Someone knocked at the door - "I'll get it," she said automatically, heading towards the door. "Good mor…ning…"

"Ah? Why is a girl like you staying at Emiya's house?" Shinji smiled thinly at her.

"…what brings you here, Matou?" she asked, unconsciously taking a step back.

"Have we met before? I would've remembered a face like that," he replied, as Shirou moved in front of Issei. "There you are, Emiya. I was coming to see if you were holding my sister hostage again-"

"Sakura's not here," Shirou interrupted curtly, feeling a strange surge of protectiveness within him. "Do you need anything else?"

"-but this is much more interesting," Shinji concluded as if he hadn't been interrupted. "Where did you find such a beautiful woman, and why are you making her answer your door? Emiya has no skill with women-"

Shirou twitched, but Issei stepped in front of him. "Let me handle this," she told him, then turned to face Shinji with a sweet smile on her face. "There are two things you should know, Matou."

Shinji was easily fooled by her expression. "Yes?"

"One," she began, "none of your fangirls know who I am."

He blinked. "And two…?"

"Two," she continued, "I no longer have to wince in sympathy whenever someone does-"

Her knee suddenly collided with his most sensitive bits. Shirou winced in sympathy as Shinji doubled over in pain.

"-that."

And she stepped back into the house and slammed the door shut.

"I've been dying to do that for years." Issei sighed contentedly. "The gender change was worth it just for that."

"…R…remind me never to make you angry, Issei," Shirou said weakly.

* * *

Obviously, Issei couldn't attend school in her current form. "Bring back the classwork and homework for me, please?" she asked. "I can't afford to fall behind for - however long this takes."

The teachers were very understanding about Issei's 'sudden illness'. The students, on the other hand, seemed divided over whether it was a legitimate sickness or he was just faking it. Most of the student council held with the former; their president would never voluntarily skip school. Shinji's fanclub agreed with his theory that it was all part of a larger plot to emasculate him.

Shirou declined to comment.

"Kuzuki-sensei, have you made any progress?" he asked when lunchtime rolled around.

Kuzuki looked up at him from over the tests he was grading. "No."

That seemed to be all that he was going to say, so Shirou coughed awkwardly before continuing the conversation. "Hey… How did you explain to Issei that his only options were the school's magi without telling him about, you know, magi?"

"He accepted that they were his only choices because I told him that they were his only choices," Kuzuki answered.

And again he returned his attention to the tests.

…Well, as they say, the unskillful talk long.

Later, Shirou put paper after paper in front of Issei. "We did this in English class, and this is the homework, and this is due two days from now in math… are you going to be able to finish all of this, Issei?" he asked after putting the last one down.

"I can manage it," Issei assured him. "At the very least, if I can't finish it tonight it will give me something to do when you're at school tomorrow."

Shirou was reassured, but was still not entirely surprised when Issei sat down across from him a few hours later, pencil and math homework in her hands. "Emiya? I don't understand this… Could you help me?"

"Of course," he answered. "What part don't you get?"

"All of it," she replied glumly. "I'm sorry to take up your time."

"Don't worry about it, Issei. I don't mind. So a derivative is a measurement of how a function changes…"

* * *

In retrospect, Shirou really needed to remember to always knock on the bathroom door. This was what, the third time?

At least this time, as soon as his brain processed that there was someone in there bathing, and he didn't have any other guys in the house so it had to be a (very naked) woman, he swiveled back around, shut the door, and stammered an apology.

"Eh? Wh-what are you apologizing for? We're both men, so it's nothing you haven't-" There was a pause while Issei remembered her sudden new gender. "…oh."

"Sorry," Shirou repeated, blushing furiously.

"I-It's okay, I'm sure it was an accident…" She cleared her throat. "Could you, um, go? I'll let you know when it's unoccupied."

He needed no further prompting, and fled in the direction of 'away from the naked woman'.

A while later Issei found him, still toweling off her hair. "It's empty. Should I have put a sign up?"

"No, it's my fault. I'm really sorry," he repeated.

"It's okay," she assured him, and then froze and blushed crimson. "U-Um… I didn't notice you until you spoke up, so how much did you…?" she asked, before continuing in a rush, "I mean I would never accuse you of anything but sometimes we have delayed reactions to these things and it's really not our fault if we don't turn around the instant we see something we shouldn't-"

"I really didn't see anything," he told her, ignoring the fact that his mind helpfully filled in the blanks for him. Issei was uncomfortable enough around him already; she didn't need to know that.

"…When I turn back into a man, can we not talk about this?" she asked. "Ever?"

Shirou nodded. "Please."

* * *

The third day of school passed uneventfully. Issei managed to conquer derivatives, and Shirou was pretty sure that she would be okay when she returned to school.

"But… I don't mean to alarm you, but what if Kuzuki-sensei can't find a way to change you back?" Sakura asked worriedly.

Issei sighed. "I don't want to have to think about that. Besides the obvious issues of me being a woman, I'd have to falsify legal documents and make it look like I was an actual woman rather than a man who suddenly changed into a woman. Someone would have to explain my sudden, permanent disappearance. And I honestly have no idea whether it would be easier to get an apartment or to explain to my father what happened."

"You know you can stay here as long as you need to, Issei," Shirou reminded her.

"But I couldn't impose on you like that for so long!" she protested. "It's bad enough as it is - you're sure there's nothing I could do to pay for my room and board?"

He shook his head. "Don't worry about it. You'd do the same thing if it were me without somewhere to stay."

"But…" Issei still looked doubtful. "That's true, but…"

"I like helping people, remember?" He gave her his most winsome smile. "Relax."

She froze and flushed red. "I… well… If you insist, Emiya."

Ilya cleared her throat. "Can I have seconds?"

"Sure." And Shirou returned his attention to the meal, allowing Issei to blush in silence.

* * *

That had been the previous day.

Today, they were watching one of Shirou's ridiculously cheesy superhero cartoons. Issei didn't care much for it, but she said that she couldn't complain about spending time with Emiya.

As the hero prepared to defeat the monster of the week through the power of his belief in justice and goodness, just as he'd done every other week, Shirou glanced over to find Issei apparently asleep on the couch.

He smiled fondly. "I know you're tired, Issei, but you'll hurt your neck sleeping like that."

She didn't stir. He clicked off the television and moved to Issei's side. "Don't make me carry you to Fuji-nee's room."

Blearily, Issei opened her eyes. "Emiya…?"

Shirou's smile widened. "I guess the show was boring, huh?"

"N-no, that's not it, I just haven't been sleeping much lately-" she protested, and then suddenly quieted down. It took Shirou a moment to realize why.

This was the first time they had been this _close_ to each other. The lights had been turned off in an attempt to make the monster at least slightly scary, but now the darkness in the room seemed much less innocent. His opinion of her appearance hadn't changed since that time he stared at her six days ago, and with her lips just slightly parted like that…

If he leaned just a little further down…

Shirou hastily backed away from her, stammering an apology. "I-I'm sorry, I don't know what came over me-!"

Issei seemed to snap out of it as well. "N-no, that's all right, I'm sure it was an accident," she squeaked. "I'm not angry with you or anything, Emiya!"

"Still… sorry," he repeated. "I, um… If you're going to fall asleep maybe you should head back to your room?"

"That, that sounds like the best idea," she agreed, rising shakily to her feet. "But I really wasn't angry with you-"

And then she was gone.

Shirou stared after her, a thought striking him. Right before he backed away - it could have been the dark playing tricks on him, but… had she been leaning up…?

* * *

"Kuzuki-sensei?" Shirou fidgeted nervously as he asked, "Have you made any progress?"

"Nothing significant," his teacher answered. Kuzuki's gaze flickered downward to a paper labeled 'effective strategies for combating teenage girls' before returning to Shirou. "I will inform you once I have found a cure."

"Yeah, I was afraid to hear that…" Shirou swallowed before continuing, "Um, so that means you probably don't know if there are side-effects of whatever happened to Issei? Like… behavior changes?"

Kuzuki frowned. "It cannot be ruled out, but I have seen no evidence for it. Why do you ask?"

"W-Well…" Shirou felt his face burning up as he continued, "She's just been acting kind of strange lately, so-"

"Before coming to conclusions, I would remember two things," Kuzuki calmly said. "Gender could have more of a mental component than previously thought, or your own impression of what is 'normal' behavior for Ryudo could be flawed."

"I know, but-"

"Judging by your observational skills, the latter is almost certainly true," he added. "Although if I was informed about the specifics of this strange behavior, I could provide a second opinion."

"…That's okay," Shirou answered, not wanting to describe how Issei might have sort of wanted to kiss him. "I was probably just imagining it, anyway. …Uh, but Kuzuki-sensei?"

"Yes?"

"What do teenage girls have to do with anything?" he asked.

"Can you think of another group more likely to change someone into a woman, had they the opportunity?" Kuzuki asked.

"Well, teenage guys, for one thing," Shirou pointed out.

Kuzuki shook his head. "…Someday, you will learn."

* * *

Apart from blushing whenever Shirou so much as looked at her - which wasn't all that strange for her - Issei seemed to be fine. So Shirou wasn't that concerned about staying up talking to her after Sakura had headed home and Ilya had gone to bed. Last night was just a one-time incident, it wouldn't happen again.

"How did Kuzuki say the search was going, Emiya?" Issei asked.

"Um, not very well…" Shirou admitted, before quickly changing the subject. "Haven't I told you that it's okay to call me my first name? We've known each other for more than two years now."

"This is the tenth time you've told me," she answered. "And my answer remains that it sounds odd to call you by anything other than Emiya. …Then again, it sounds odd to say anything when I'm like this," she admitted.

"Then you could try switching for as long as this lasts," he suggested.

"…If I must, Shirou…" She paused before shaking her head. "That's even worse."

He sighed. "It was worth a try."

"So… no luck with the search? I wonder if I should have let Matou and Einzbern buy me that school uniform…" Issei mused.

"Just because we haven't found anything out in the first week doesn't mean you're stuck like this!" he protested.

"I know, but the possibility is there. And I… I'm not looking forward to being a woman for the rest of my life," she admitted. "I mean, I'll have to fake my own death and wear women's clothes all the time and act like a woman and men will start to flirt with me and my family will go into mourning and-"

"Issei." Gently, Shirou grabbed her shoulders. "We'll get you back to normal no matter how long it takes. I promise."

"…you're right as always, Emiya." She sighed in relief before offering her friend a small smile. "I don't know how I would have gotten through this without you. If there's anything I can do…"

He really needed to stop getting so close to Issei.

"…anything to repay you for all of this…"

Issei really needed to stop being so pretty.

"…please let me know. I'd be willing to - no, I _want_ to…"

So it really wasn't Shirou's fault that he had to lean down and kiss her.

He felt her stiffen slightly in surprise before relaxing. Her lips, just as soft as they'd appeared, slowly started to move against his. Her eyes fluttered closed as she rested her hands on his cheeks and gently pulled him in closer-

Something about the motion reminded him of what exactly he was doing, and Shirou broke apart from her. "I, I'm sorry, Issei!" he gasped, taking a few steps back. "I'm so sorry, I shouldn't have-!"

She opened her eyes. "Wh-why are you apologizing?" she asked, sounding more disappointed than anything.

His words came out in a rush. "Because we don't know exactly what happened to you and maybe there's something that's making you like me back because I know you don't when you're a guy and if there is something making you do this then this would practically be like forcing you and I could never take advantage of you like that!"

Issei stared at him for a few seconds. "Take… advantage… of me?" she asked.

He nodded helplessly.

She continued staring for another moment, then burst into laughter.

"H-hey, I'm being serious!" he protested.

"E…Emiya, you magnificent idiot…" she gasped out between peals of laughter. "Take _advantage_ of me? I _don't _like you back? Only you could think that way, Emiya."

"But…" Shirou frowned at her. "You've never…"

"Because you don't usually like me that way and it would be pointless," she answered automatically. Her snickers gradually died down, and she got to her feet. Taking a few steps until she was right in front of Shirou, she whispered, "And once I'm back to normal, you won't. So if I ever wanted to do anything… now would be the time, wouldn't it?"

"Issei… I don't know that this isn't the spell talking, or whatever happened to you," he pointed out.

Her smile didn't fade. "I guess you don't. If I give you a hard time about it when I change back, just remind me that I said I'd do anything to repay you, all right? And if this is really what I want, then… please…"

It was difficult to say no to someone with that hopeful, almost pleading look in their eyes. "…All right."

Her entire face seemed to light up as she leaned in for another try.

* * *

Issei woke a few hours before he had to. Being a woman was terrible for his sleep schedule, he thought miserably.

He was about to roll over and go to sleep when he noticed that he no longer had to avoid squishing the new additions to his chest.

…He was a man again? Just like that? Without any ridiculously long quest to find who did this to him and why? No, but maybe Kuzuki had accomplished something after all…

He sat up and looked down at himself.

Flat chest. Short hair. Completely and utterly male.

Issei felt a grin spreading across his face as he turned to tell Shirou the news.

Shirou was still fast asleep. His hair was tousled, and his face wore a peaceful expression that Issei had rarely seen on him while he was awake. Unconsciously, he groped around for the suddenly missing source of body heat.

This… would be the last time Issei got to see him like this, probably. They weren't quite at the sleep-in-the-same-bed point, and anyway, it would be awkward now that they'd… done this.

Suddenly, Issei really didn't want to run the risk of waking Shirou up.

He watched his friend sleep for a few more moments, then curled back up next to him. "Pleasant dreams, Emiya," he whispered.

Some time after that and before Shirou woke up, a white-haired girl stepped into the room, intent on demanding to know why her big brother hadn't made breakfast yet. Instead, she blinked at the two boys tangled together underneath the sheets.

Ilya suddenly wished that she had a camera. Blackmail opportunities like this didn't happen every day, you know.

* * *

Shirou's awakening was less peaceful and more filled with panicked thoughts of 'there is a naked man in my bed! …besides me!'

After they had both gotten dressed, he cleared his throat. "I'm really glad you're okay, Issei."

"So am I," Issei replied, although he sounded somewhat regretful. "Once again, I - thank you for having me here, and I-"

Distantly, they heard the doorbell ring.

"…If it's Kuzuki-sensei, I should get it," Issei said, rising to his feet. "Hold on a moment."

Shirou stared after him for a few moments before sighing and following him to the entrance of the house.

It was, in fact, Kuzuki. "I see you have reverted, Ryudo. Do you know what caused you to return to your usual gender?" he asked.

Issei flushed. "I… well, that is… It was probably just the time on it running out or something, but…"

"I see." Kuzuki looked over at Shirou, and more specifically Shirou's disheveled appearance, before returning his gaze to Issei, and more specifically Issei's similarly messy state. If he had filled in the gaps, he didn't show it. "Are you certain that you have reverted completely? There are no side effects of the time you spent as a woman?"

"I don't think so," Issei answered, looking over at Shirou. "Have you noticed anything?"

"Not real-" Shirou paused, thoughtfully looking over at Issei. "…have you always been that attractive?" he asked in genuine curiosity.

Issei turned bright red and turned back to Kuzuki so quickly that Shirou thought he was going to have whiplash. "O-o-one side effect."

"Ah. I will continue looking into the cause of this-"

"Um, Kuzuki-sensei…" Issei interrupted with a quavering voice. "I… well… You're busy, aren't you? You don't really have to… it's not that important to…" He trailed off.

"…I see." The corners of Kuzuki's mouth twitched up into an almost unnoticeable smile. "I will refrain from attempting to remove this side effect for the time being, then."

Issei breathed a sigh of relief. "Thank you, Kuzuki-sensei."

"Um, don't I get a say in this…?" Shirou asked worriedly.

"…I suppose…" Issei admitted. His smile disappeared. "I mean, of course you would prefer not to…"

It had to be purely the side effects that made Shirou's heart sink at that crushed expression on his friend's face. Had to be. "A…actually, never mind."

* * *

(Several weeks later…)

"Emiya? Do you have some free time?" Issei asked.

Shirou nodded.

"I was hoping you would help me determine something…" And then Issei kissed him.

The side effects were still there. Great. "I-Issei, what brought this on?"

"Greek mythology," Issei answered matter-of-factly, pulling away from Shirou in order to suck on his ear.

"…Huh?" Shirou asked, trying very hard not to think about how ridiculously good that felt.

And those were Issei's hands unbuttoning his jacket, weren't they? "There's this myth - I forget the name of the person, but apparently he had spent time as both a man and a woman, and someone asked him who had more pleasure from coupling." Issei ran his tongue over the exposed parts of Shirou's neck before continuing. "His answer was that the woman received ten times more pleasure than the man did. But I don't even know if this person existed at all, and even if he did, I don't know if he had really spent those years as a woman, so…"

Shirou thought about this for a moment. "…you want me to sleep with you for research?"

"That was the idea." And Shirou wondered why Issei seemed to be skipping over his shirt entirely, in favor of other articles of clothing. "You don't mind, do you? I-If you do, I understand…" Issei added quickly, his nervousness obvious.

"It's okay," Shirou answered a little too quickly. The side effects were _definitely_ still there. He tried to play them down, all too aware of the blush on his face. "I mean, if it's for research…"

Issei beamed at him. "You're an invaluable assistant to the progress of knowledge in our society, Emiya."

Then he gracefully sank to his knees.

"H-hey, what does that have to do with the re- nngh! …K…keep going…"


End file.
